Cryogenic Cooling Method and Installation Using Liquid CO2 and Employing Two Exchangers in Series

ABSTRACT

A method and installation using liquid CO 2  as a cryogenic fluid for transferring cold energy to products, wherein liquid CO 2  is sent into a heat exchanger system including first and second heat exchangers connected in series. The liquid CO2 is supplied to the first heat exchanger where it evaporates. Gaseous CO2 from the first exchanger is supplied to the second heat exchanger. The atmosphere surrounding the products is blown across the heat exchangers to provide a cooled atmosphere with which to cool the products. The latent heat of vaporization of the liquid CO2 in the first heat exchanger and the sensible heat of the gaseous CO2 in the second heat exchanger satisfies the cooling requirement of the products. The first exchanger is kept at a pressure higher than the triple point pressure for CO 2 , whereas the second exchanger is held at atmospheric pressure or at a pressure between the triple point of the fluid and atmospheric pressure.

The present invention relates to the field of cooling processes employing CO₂.

The use of CO₂ in such cooling processes is, as is known, very advantageous, since this fluid has a solid phase at −80° C. at atmospheric pressure, thereby allowing dry ice to be used in certain applications, this ice being very effective, especially as far as localized cooling without refrigeration equipment is concerned. Solid CO₂ has many applications, such as for example dry ice bags, which are loaded into containers for transporting food or pharmaceutical products, or which indeed may be used to keep meals cool in the field of aerial transportation.

However, when this gas is used in (“indirect injection”) heat exchangers, typically tube or finned heat exchangers, this advantage turns into a drawback because the untimely appearance of solid CO₂ (carbon snow) in an exchanger rather rapidly leads to the latter becoming blocked.

Therefore, to prevent this drawback of transition into the solid phase, it is sought to prevent the solid phase of CO₂ from appearing, and conditions are therefore preferred that make it possible to keep the CO₂ in liquid or gaseous form in the entire exchanger.

In order to exchange heat while ensuring the CO₂ remains in its gaseous or liquid phase (vaporization of the liquid CO₂ without running the risk of forming solid CO₂), the pressure in the tube must be kept above the theoretical pressure of 5.18 bar corresponding to the triple point pressure of this fluid. In practice, the system is restricted as it were to a pressure of 6 to 7 bar, thus ensuring a safety margin of 0.82 to 1.82 bar.

Whereas the sublimation temperature of solid CO₂ at atmospheric pressure is −80° C., keeping the pressure in the exchanger at 6 bar relative increases the vaporization temperature to about −50° C.

However, carrying out the heat exchange at 6 bar and not at atmospheric pressure slightly decreases the cooling capacity of the CO₂. Specifically, when a kilogram of CO₂ is taken out of storage, for example under standard conditions such as 20 bar absolute/−20° C., and enters into an exchanger, it releases 277.97 kJ/kg if it is discharged at −50° C. in gaseous form at 6 bar relative, whereas the same amount of CO₂ releases 292.6 kJ/kg when it is discharged at −50° C. at atmospheric pressure, i.e. an increase of 5%.

By way of example, CO₂ discharged at 6 bar after passing through an exchanger is used in applications such as refrigerated trucks but also in freezing tunnels or chambers. In these applications, a heat exchanger is supplied with liquid CO₂ which, by evaporating in this exchanger, extracts heat from the medium to be cooled and thus produces the desired cooling. Cooling of the products to be cooled is achieved via heat exchange with the internal air of the tunnel, chamber or truck, via blowing means associated with each exchanger.

It will therefore be understood that it would be advantageous to be able to provide a technical solution allowing heat exchange in a tube or finned heat exchanger (indirect exchange), at exchanger temperatures that are nonetheless low (typically −50° C.) without of course running the risk of forming snow and losing the cooling capacity of the CO₂ following its expansion from 6 bar to atmospheric pressure.

As will be seen in more detail below, the present invention provides a new exchange solution, the main features of which may be summarized as follows:

-   -   the solution proposed here relates to the exchanger         configuration adopted, the exchanger used, which may for example         be a tube or finned heat exchanger, consisting of two exchangers         connected in series;     -   the first exchanger can be supplied with liquid CO₂ (for example         under standard conditions such as −20° C./20 bar), the liquid         encountering, before it enters the first exchanger, a         thermostatic regulator or a temperature probe/regulator/valve         assembly, or any other means allowing the flow rate of the CO₂         entering the 1st exchanger to be matched to the cooling         requirements in question, i.e. allowing the “excess”         temperature, or, in other words, the temperature difference         between the temperature corresponding to the saturation vapor         pressure (for example at 6 bar, −53.1° C.) and for example −50°         C., which corresponds to 3.1° of excess, to be controlled;     -   the pressure in the first exchanger is kept above 5.18 bar         relative, i.e. the temperature of the phase change of the CO₂         (the formation of snow thus being prevented), the pressure is,         for example, kept at 6 bar absolute by virtue of a back-pressure         regulator or a pressure sensor/regulator/valve assembly, placed         at the outlet of the first exchanger;     -   this arrangement makes it possible to ensure that the CO₂ in the         first exchanger is only present strictly in its gas/liquid         two-phase form without at any point the conditions used allowing         a solid to form;     -   the minimum temperature obtained in this first exchanger is then         about −50° C.; and     -   according to one embodiment, the back-pressure regulator at the         outlet of the first exchanger is preceded by a phase separator         in order to prevent any liquid from exiting the first exchanger.

The back-pressure regulator and the outlet of the first exchanger will possibly be installed in an upper part of the complete installation to prevent liquid from escaping, but configurations where the two exchangers are located at the same level may also be envisioned.

The optional presence of the aforementioned separator increases the reliability of the system. It prevents liquid from reaching the back-pressure regulator and therefore snow from forming and causing a blockage at this point.

-   -   In summary, the liquid CO₂ vaporizes in the first exchanger and         the gas formed in this exchanger, for example at 6 bar, is         released in the second exchanger;     -   this second exchanger is at atmospheric pressure (or in any case         at a pressure below the triple point of the fluid), the gas then         expands, after entering this second exchanger at 6 bar (or more         generally at the pressure maintained in the first exchanger), to         atmospheric pressure (or in any case to a pressure between the         triple point of the fluid and atmospheric pressure) which causes         cooling, namely to a temperature typically between −60° C. and         −70° C.;     -   and therein lies the advantage of the present invention, since         the cooling produced by expansion to atmospheric pressure is         used by the second exchanger, and thus all the energy contained         in the CO₂ is used.

The present invention thus relates to a process employing liquid CO₂ as a cryogenic fluid for cooling products, this process being an “indirect injection” cooling process where the liquid CO₂ is transferred to a heat exchanging system where it evaporates, the removal of heat from the products occurring by an exchange between the atmosphere surrounding the products and the cold walls of the heat exchanger, this process being noteworthy in that the exchanging system consists of two exchangers connected in series, the first exchanger being kept at a pressure above the triple point pressure of CO₂, whereas the second exchanger is itself kept at atmospheric pressure or at a pressure between the triple point of the fluid and atmospheric pressure.

The present invention also relates to an installation for cooling products using liquid CO₂, the installation employing an “indirect injection” process, and comprising:

-   -   a heat exchanging system through which the liquid CO₂ can pass;         and     -   blowing means associated with the heat exchanging system, able         to bring the atmosphere surrounding the products into contact         with cold walls of the heat exchanging system,

the installation being noteworthy in that the following measures are implemented:

-   -   the exchanging system consists of two exchangers connected in         series;     -   the installation comprises, upstream of the inlet of the first         exchanger, a means able to adjust the CO₂ flow rate and to         control the excess temperature thereof relative to the         corresponding temperature at saturated vapor pressure, such as a         thermostatic regulator or a temperature probe/regulator/valve         assembly;     -   the installation comprises a means for keeping the pressure in         the first exchanger at a pressure above the triple point         pressure of CO₂, preferably a back-pressure regulator or a         pressure sensor/regulator/valve assembly; and     -   the second exchanger is at atmospheric pressure or at a pressure         between the triple point of the fluid and atmospheric pressure.

The installation therefore comprises, if required, a means for keeping the pressure in the second exchanger at atmospheric pressure or at a pressure between the triple point of the fluid and atmospheric pressure.

Other features and advantages of the present invention will become more clearly apparent from the following description given by way of completely nonlimiting illustration with regard to appended FIGS. 1 and 2, which are partial schematic representations of installations according to the invention, FIG. 3 showing the temperature profile expected in the exchanger assembly on the CO₂ and heat-transfer fluid (air) sides.

The following elements, and therefore the course followed by the CO₂, in its various phases, in the installation, may be seen in FIG. 1:

-   -   the first exchanger is able to be supplied with liquid CO₂ (for         example under standard conditions such as −20° C./20 bar), the         liquid encountering, before it enters the first exchanger, a         thermostatic regulator (downstream of point 1) or any other         means allowing the flow rate of the CO₂ reaching the first         exchanger to be matched to the cooling requirements in question,         i.e. to control the excess temperature, or, in other words, the         temperature difference between the temperature corresponding to         the saturated vapor pressure (for example 6 bar, −53.1° C.) and         for example −50° C., which corresponds to 3.1° C. of excess         temperature.

Downstream of point 2 the fluid enters into the first exchanger.

-   -   a pressure higher than 5.18 bar relative is maintained in this         first exchanger, temperature of the phase change of CO₂ (thus         allowing the formation of snow to be prevented), by virtue of         the back-pressure regulator placed at the outlet of this first         exchanger in the figure (back-pressure regulator placed between         points 3 and 4);     -   this arrangement makes it possible to ensure that the CO₂ in the         first exchanger is present only in a strictly liquid/gas         two-phase form, without at any point the conditions used         allowing a solid to form;     -   the minimum temperature obtained in this first exchanger is then         −50° C.;     -   in the embodiment illustrated here, the back-pressure regulator         at the outlet of the first exchanger is preceded by a phase         separator (between points 3′ and 3), in order to prevent any         escape of liquid from the first exchanger. In this embodiment,         the back-pressure regulator and the outlet of the first         exchanger are installed in an upper part of the complete         installation, to prevent escape of liquid.     -   at the outlet of point 4 and therefore of the back-pressure         regulator, the gas enters into the second exchanger, which it         exits at point 5.

The table below collates the thermodynamic properties of the fluid at various points in FIG. 1, and makes it possible to show unambiguously the advantages of the invention in terms of cooling efficiency. The table especially illustrates a number of temperature conditions, at the outlets of the exchangers.

In addition, to clearly show the benefit of the present invention, the energy efficiency of a system not employing the invention and a system employing the present invention are compared, in the case where the final temperature in the exchanger is −25° C. and in the case where the final temperature in the exchanger is −5° C.

Considering the first case (the final temperature in the exchanger being −25° C.)

-   -   for a system employing a single exchanger: 1 kg of CO₂ releases         457−154.5=302.5 kJ;     -   for a system employing two exchangers according to the         invention: 1 kg of CO₂ releases 464.5−154.5=310 kJ; i.e. a 2.5%         increase in energy.

In the second illustrative case, where the final temperature of the exchanger is −5° C.:

-   -   for a system employing a single exchanger: 1 kg of CO₂ releases         474.6−154.5=320.1 kJ;     -   for a system employing two exchangers according to the         invention: 1 kg of CO₂ releases 480.8−154.5=326.3 kJ; i.e. a         1.9% increase in energy.

TABLE 1 Point on the figure T (° C.) P (bar abs) H (kJ/kg) 1 −20.0 19.7 154.5 2 −53.1 6 154.5  3′ −53.1 6 431.6 3 −50.0 6 434.5 3 standard −25.0 6.0 457.0 3 standard −5.0 6.0 474.6 4 −63.1 1 434.5 5 −25.0 1 464.5 5 −5.0 1 480.8

The temperature profile expected in the exchanger on the CO₂ and heat-transfer fluid (air for example in an application such as the transportation of frozen products) sides, shown in FIG. 3, demonstrates that the present invention also has a beneficial impact on the temperature profile in the exchangers: the fact that the second exchanger stage is at atmospheric pressure makes it possible to take advantage of a cryogenic effect as the curves in FIG. 3 show.

If FIG. 1 showed a first embodiment of the invention, FIG. 2, for its part, shows another, which will not be described in greater detail here. As will become clear on examination of the figure, it illustrates a variant employing:

-   -   upstream of the first exchanger, an assembly consisting of a         calibrated orifice and a temperature-controlled valve, rather         than a thermostatic regulator; and     -   at the outlet of the first exchanger of the installation, a         pressure sensor/regulator/valve assembly, rather than a         back-pressure regulator. 

1-9. (canceled)
 10. A process for indirectly cooling products using liquid CO₂, comprising the steps of: supplying liquid CO₂ to a heat exchanging system at which the liquid CO₂ evaporates, the heat exchanging system comprising first and second heat exchangers connected in series, the first heat exchanger being kept at a pressure above the triple point pressure of CO₂, the second heat exchanger being kept at atmospheric pressure or at a pressure between the triple point pressure of CO₂ and atmospheric pressure; and removing heat from the products through heat exchange between cold walls of the heat exchangers and an atmosphere surrounding the products.
 11. The process of claim 10, further comprising: adjusting a flow rate of the liquid CO₂ supplied to the first heat exchanger with a liquid CO₂ flow rate adjustment element disposed upstream of the first heat exchanger, the adjustment being based upon a cooling requirement of the products to be cooled, the liquid CO₂ flow rate adjustment element comprising either a thermostatic regulator or a temperature probe/regulator/valve assembly; maintaining a pressure in the first heat exchanger above the triple point pressure of CO₂; and transferring gaseous CO₂, formed from vaporization of the liquid CO₂ in the first heat exchanger, to the second heat exchanger
 12. The process of claim 11, wherein the pressure in the first heat exchanger is kept at a pressure above the triple point pressure of CO₂ by virtue of a back-pressure regulation device that is placed at an outlet of the first heat exchanger, the back-pressure regulation device comprising either a back-pressure regulator or a pressure sensor/regulator/valve assembly.
 13. The process of claim 12, wherein a phase separator is disposed in fluid communication between the outlet of the first heat exchanger and the back-pressure regulation device, the phase separator preventing liquid CO₂ from flowing into the second heat exchanger.
 14. An installation for cooling products using liquid CO₂, comprising, a first heat exchanger receiving liquid CO₂, a second heat exchanger receiving gaseous CO₂ formed from vaporization of the liquid CO₂ in the first heat exchanger, a blower operatively associated with the first and second heat exchangers, a liquid CO₂ flow rate adjustment element disposed upstream of the first heat exchanger, and a back-pressure regulation device, wherein: the liquid CO₂ flow rate adjustment element is adapted and configured to adjust a flow rate of liquid CO₂ into the first heat exchanger based upon a cooling requirement of the products to be cooled and comprises either a thermostatic regulator or a temperature probe/regulator/valve assembly; the blower is adapted and configured to blow an atmosphere surrounding the products to be cooled into contact with cold walls of the heat exchange system; the first and second heat exchangers are connected in series; the back-pressure regulation device is disposed downstream of an outlet of the first heat exchanger and is adapted and configured to maintain a pressure in the first heat exchanger above the triple point pressure of CO₂.
 15. The installation of claim 14, further comprising a phase separator inserted upstream of the back-pressure regulation device.
 16. The installation of claim 14, wherein the back-pressure regulation device comprises either a back-pressure regulator or a pressure sensor/regulator/valve assembly. 